The present invention relates to refractory compositions. More particularly, this invention relates to a particulate refractory composition which is suitable for casting into large blocks that are resistant to the corrosive attack of molten metal.
In the processing of metal such as aluminum and aluminum alloys for casting or purification, ingots of the metal are generally melted in a furnace which is lined with refractory material. This material must not react with the molten metal, but must contain it without dissipating its heat. Thus, the refractory material must exhibit low thermal conductivity and resistance to metal attack (by chemical reaction or by wetting). The material should also exhibit dimensional stability, ease of machinability, spall resistance and resistance to thermal and mechanical shock.
The particular furnace application will dictate the properties and characteristics that are desired in the furnace lining. Such properties and characteristics depend on the type of lining construction utilized and on the particular refractory composition employed in the lining.
Conventional aluminum melting and holding furnaces are typically lined with refractory bricks joined together with mortar. These mortared brick constructions provide high strength, but are subject to failure caused by thermal shock. In addition, it is difficult to line non-planar surfaces with a brick and mortar construction; hence furnace design flexibility is reduced. Finally, mortared brick constructions are expensive to install and maintain, and the many mortared joints of such constructions provide sites for thermal stress cracks and consequent metal penetration and lining failure.
In order to eliminate some of the disadvantages of mortared brick linings, monolithic linings have been proposed as alternatives. Such linings are applied to the furnace walls by ramming or tamping, and then are fired in place by heating of the furnace. These linings provide the advantage of no-joint construction and enhanced furnace design flexibility. However, since temperature conditions are rarely uniform throughout the furnace, fired properties may not be uniform throughout the lining. This may result in areas of the lining having reduced strength and in cracking in the lining because of uneven thermal expansion and shrinkage. The presence of cracks in the lining invites metal penetration therethrough and consequent lining failure.
It has been suggested that the principal disadvantages of both mortared brick and monolithic constructions may be avoided while the principal advantages of each may be retained in a large block construction. Large blocks should be large enough to eliminate the need for many mortared joints, yet small enough to be formed and fired outside of the melting furnace. Generally, the practical size of such blocks has been limited by the weight that can be handled with relative ease. Such blocks have therefore been constructed with weights near 2,000 kg.
It has already been explained that the properties and characteristics desired in a furnace lining depend not only on the type of lining construction utilized, but also on the refractory composition employed in the lining. If a brick and mortar construction is desired, a variety of refractory brick and mortar compositions is commercially available. Similarly, for monolithic or large block constructions, a variety of refractory compositions has been employed. Generally, however, the refractory compositions which have been employed in monolithic or large block constructions in metal melting furnaces have been either phosphate bond-forming plastic materials or refractory concretes.
The phosphate bond-forming plastic refractories are generally comprised of a blend of ground refractory materials and phosphoric acid. The blend is supplied in plastic form, suitable for ramming or tamping into the desired shape. The refractory concretes, on the other hand, are generally comprised of a blend of ground refractory materials and a bonding or cementing agent such as calcium aluminate. The blend is mixed with water and generally poured into a mold for casting. The principal difference, however, between phosphate bond-forming plastic refractories and refractory concretes is in their bonding mechanisms.
Phosphate bond-forming plastic refractories are characterized by the formation of phosphate bonds between constituents of the refractory aggregate. For a plastic refractory containing alumina, the initial bonding reaction: EQU Al.sub.2 O.sub.3 +6H.sub.3 PO.sub.4 .fwdarw.2Al(H.sub.2 PO.sub.4).sub.3 +3H.sub.2 O,
proceeds upon exposure of the blend to air or upon the application of heat.
When the blend is subsequently heated to 320.degree.-400.degree. C., the intermediate compound, Al(H.sub.2 PO.sub.4).sub.3, reacts with additional alumina to form AlPO.sub.4 by the following reaction: EQU Al(H.sub.2 PO.sub.4).sub.3 +Al.sub.2 O.sub.3 .fwdarw.3AlPO.sub.4 +3H.sub.2 O.
A phosphate bond-forming plastic refractory blend which has been produced by A. P. Green Refractory Company includes a refractory aggregate comprised of about 86.7% by weight Al.sub.2 O.sub.3, 9.8% by weight SiO.sub.2, 2.2% by weight TiO.sub.2, 1.2% by weight Fe.sub.2 O.sub.3 and traces of CaO, Na.sub.2 O and K.sub.2 O. To this aggregate is added sufficient phosphoric acid to provide a P.sub.2 O.sub.5 addition of about 4.8% by weight.
Refractory concretes are characterized by the formation of hydraulic cement bonds between the constituents of the refractory aggregate upon mixture of the blend with water. These concretes are further characterized by the dissolution of these hydraulic bonds upon heating and the subsequent formation of ceramic bonds between the constituents of the aggregate as heating continues. In refractory concrete blends containing alumina and calcium aluminate, the initial bonding derives primarily from the hydration of CaO.Al.sub.2 O.sub.3 to CaO.Al.sub.2 O.sub.3.10H.sub.2 O and 2CaO.Al.sub.2 O.sub.3.8H.sub.2 O. These metastable crystalline hydrates spontaneously change to the more stable 3CaO.Al.sub.2 O.sub.3.6H.sub.2 O, envolving physically absorbed water and Al.sub.2 O.sub.3.3H.sub.2 O as the change occurs. Upon heating to elevated temperatures (200.degree.-350.degree. C.), however, these hydrates begin to lose appreciable portions of their combined water. As combined water is lost, the strength of the hydraulic bonds between the constituents of the aggregate is reduced, and the predominance of CaO.Al.sub.2 O.sub.3 and CaO.2Al.sub.2 O.sub.3 increases. As heating continues above about 1100.degree. C., the concrete develops an appreciable ceramic strength by reason of the melting of some of the low melting point compounds in the cement which combine with the aggregate to form a ceramic bond. Ceramic bond strength increases upon firing to 1400.degree.-1650.degree. C. with the appearance of appreciable quantities of CaO.6Al.sub.2 O.sub.3.
A refractory concrete composition of the type described above is disclosed in U.S. Pat. No. 2,516,892 of Lobaugh. This composition contains 15-93.5% by weight refractory aggregate such as fireclay grog, crushed firebrick, expanded shale, diatomaceous earth, vermiculite, crushed red brick, and the like, 5-60% by weight calcium aluminate and 0.5-25% by weight of a substantially insoluble silicate frit which has a melting point of 1600.degree. F. or less. The frits which may be used in this composition are smelted mixtures of soluble and insoluble inorganic materials which are prepared by melting the soluble materials in the presence of sufficient silica and at a temperature high enough to form substantially insoluble silicates.
Another refractory composition is disclosed in U.S. Pat. No. 3,269,849 of Caprio et al. This composition contains 20-30% by weight of a fibrous material such as fibrous alumina-silica or fibrous potassium titanate, 40-60% by weight asbestos fiber, 20-25% by weight calcium aluminate and 2.5-5% by weight cryolite (Na.sub.3 AlF.sub.6).
U.S. Pat. No. 4,158,568 of LaBar also describes a refractory concrete composition of the type described generally herein. This composition contains 60-88 parts by weight alumina, 10-34 parts by weight calcium aluminate and 1.5-10 parts by weight of a zinc borosilicate frit which consists essentially of 50-60% by weight zinc oxide, 20-40% by weight boron oxide, 8-12% by weight silicon dioxide and 0-10% by weight aluminum oxide.
Although the known refractory compositions may be satisfactorily employed in many furnace lining applications, few are suitable for use in an aluminum/aluminum alloy melting furnace. For example, the phosphate bond-forming materials are generally considered to be somewhat unsuitable for such use because of the tendency of molten aluminum to react with AlPO.sub.4. Similarly, the refractory concretes which contain silicon dioxide in their aggregates are considered to be generally unsuitable for use in the linings of aluminum/aluminum alloy melting furnaces because of the tendency of aluminum to reduce the silicon dioxide.
Previous efforts aimed at developing a suitable refractory concrete composition for use in an aluminum/aluminum alloy melting furnace have combined large amounts of calcium aluminate and boron oxide and zince oxide additives in order to provide ceramic bond strength and resistance to the corrosive attack of aluminum. However, the refractory blocks produced from such compositions are subject to load deformation at temperatures within the operating range of aluminum/aluminum alloy melting furnaces (up to 900.degree. C.). Such blocks may crack under the weight of higher blocks at such temperatures, allowing metal penetration through the lining and consequent lining failure. These blocks are also subject to explosive spalling because of their low surface porosity and the consequent inability of absorbed water to be safely released upon heating.